Ghost reflections in a catadioptric scatterometer objective are excluded from an angle-resolved spectrum measurement by using a partial pupil for illumination and for the measurement excluding the area of the pupil plane that has been illuminated. Ghost reflections are reflected back into same point in the pupil plane. The ghost reflections do not interfere with the signal in the non-illuminated area of the pupil plane. An illumination system provides a beam of electromagnetic radiation to illuminate a first area in an illumination pupil plane of the objective. The objective is arranged as to illuminate the substrate with the beam of electromagnetic radiation. The illumination pupil plane is the back projected image of the pupil plane of the objective and is also imaged into the measurement pupil plane at the back focal plane of the objective, via auxiliary optics. A detector is configured to measure an angle resolved spectrum arising from the illumination of the substrate, in a measurement area of the measurement pupil plane of the objective excluding an area corresponding to the first area.